A vertical seismic profile (VSP) is a technique of seismic measurements with down-hole receivers. VSP data usually are obtained by generating one or more shots from a seismic source located in one or more selected positions on the surface. The signal produced by each shot is detected at locations along a borehole extending into the formation by multiple receivers, or by a group of receivers that is moved along the borehole. The primary goal of a VSP is obtaining the subsurface reflectivity (which gives information about geological formations) with high vertical resolution, which may be achieved because the frequency content of VSP data is usually higher than that of surface seismic. Other applications include generating an image of the subsurface area that is poorly illuminated with surface seismic, recovery of attenuation, and reservoir characterization.
The method called reflection imaging allows one to get the reflectivity distribution in the subsurface. The success of reflection imaging strongly depends on the velocity model assumed during migration of seismic data (the process by which the seismograms in the time domain are mapped into subsurface images in the depth domain). For VSP imaging, a velocity model retrieved from surface seismic is typically used. But due to lower resolution and overburden complexities, this model may be inaccurate. Inaccurate velocity models smear reflection events in VSP images and result in mismatches between VSP and surface seismic images. The velocity model above the borehole receivers may be improved by first break travel time tomography of the VSP data itself. It is difficult to improve the velocity model below the borehole receivers by conventional velocity analysis or reflection tomography, however, because of the very limited range of incident wave angles in the VSP data. Thus, it is desirable to provide a method that can be used to recover accurately the velocity model for portions of the formation below the downhole receiver(s).